1. Field of the Invention
The present invention relates generally to dock levelers. More particularly, the present invention relates to an improved holddown mechanism or holddown for use with mechanical dock levelers.
2. Description of the Related Art
Dock levelers are widely used in conjunction with freight vehicle loading docks and the like to provide a continuous path between a loading dock and the load bed of a vehicle. One widely used type of dock leveler comprises a generally planar ramp or deck member which is pivotally connected to a frame disposed in a pit or recess in a loading dock and moveable between a stored position substantially coplanar with the surface of the loading dock and an elevated position preparatory to engagement with the load bed of a vehicle when it is situated in proximity to the loading dock. So-called mechanical dock levelers typically utilize one or more heavy duty springs or other means which operate to bias the dock leveler's ramp from its stored position to an elevated position, including the engaged or working position of the dock leveler's ramp with respect to the vehicle load bed.
Dock levelers having upwardly biased ramps typically also include a holddown mechanism or one-way locking device that, when it is engaged, prevents the upward movement of the ramp while allowing its downward movement. Releasing this device allows the free end of the dock leveler's ramp to pivot upward about the ramp's hinge point.
Such holddown mechanisms typically includes a first member that is pivotally interconnected with the frame of the dock leveler and a second member that is extendible and retractable relative to the first member and pivotally interconnected with the dock leveler's ramp. A releasable engagement arrangement, such as a linear ratchet and pawl, is interposed between these members for selectively fixing their relative positions and thereby one-way locking the position of the ramp relative to the frame of the dock leveler and therefore the loading dock. See U.S. Pat. Nos. 6,061,859, 3,967,337, 3,699,601, 3,579,696, 3,137,017, and 3,117,332.
All “linear ratchet and pawl” type holddowns have similar construction in which a ratchet bar telescopes inside a housing. The end of the ratchet bar is attached to either the bottom of the dock leveler's ramp or its base frame. Enclosed within the housing is a pawl that engages the ratchet and effectively prevents the upward movement of the pivoting ramp's free end. The ratchet is released via linkages and a manually tensioned chain. Connected to the housing is a compression spring that enables the holddown to allow for some limited upward movement of a pivoting ramp's free end to accommodate for those situations such as when the shock-absorbing springs on a being-unloaded vehicle cause its load bed to push up on the bottom of a dock leveler's ramp whose holddown is engaged or not released.
“Circular ratchet and pawl” type holddowns have also been utilized. See those described in U.S. Pat. Nos. 4,703,534 and 4,126,909, which use, respectively, either an extension or torsion spring to enable such holddowns to allow for limited upward ramp movements to accommodate limited upward movements of a vehicle load bed. See also the “circular drum” setup in U.S. Pat. No. 3,316,575 which uses a friction joint in place of a “ratchet and pawl” and an extension spring to enable this holddown to allow for some limited upward ramp movements. Linear friction brake holddowns have also been developed, see U.S. Pat. No. 5,303,443.
All of these prior art holddowns can have operational problems and/or be costly to manufacture. For example, those which utilize some type of spring to allow the holddown to “give” so as to accommodate limited upward movements of a vehicle load bed can encounter operational problems when their dock levelers are used by air cushioned trucks which can have significant load bed movements—such upwards movements can bottom out the holddown's spring (in the compression spring case) or over-extend its spring (in the extension spring case) and jam the holddown or damage its parts. To help avoid these problems, it would be advantageous if holddowns had the capability to be self-releasing in these situations.
Meanwhile, the “ratchet and pawl” engagement arrangements of these holddowns can be costly to manufacture. The ratchet, in particular, is a highly machined and therefore expensive part. The ratchet and pawl are also subject to wear (e.g., the teeth on the ratchet bar wear down during normal use and can necessitate replacement of the bar), require adequate lubrication, and are easily damaged if the spring (in the more common compression spring setup) bottoms out. Wear and damage to holddowns are often a limiting consideration is establishing the length of warranty that is provided on a mechanical dock leveler.
Thus, there is the need for the development of an improved holddown, for mechanical dock levelers, which will both last longer, as a result of having fewer operational problems, and cost less to manufacture, while also having a self-releasing capability in situations where significant load bed movements or elevation changes are encountered.